Spark-ignition engine for lean air-fuel mixture

ABSTRACT

A spark-ignition-type internal combustion engine having a main combustion chamber, an auxiliary combustion chamber, and a spark plug disposed in the passage interconnecting those chambers and wherein the spark plug is positioned in an enlarged portion of the passage.

This invention relates to an improvement in internal combustion enginesof the type that have an auxiliary combustion chamber, to achieve betteroperation of the engines with lean air-fuel mixtures.

It is known to operate an internal combustion engine with a leanair-fuel mixture, in which the ratio of air to fuel is higher than thetheoretical value, and that this is effective to decrease suchdetrimental exhaust gas components as CO (carbon monoxide), HC(hydrocarbons) and NOX (nitrogen oxides). But a lean air-fuel mixture,in general, is inferior in ignitability, and, when ignited, is slow tocombust. Accordingly, a lean air-fuel mixture is apt to cause misfiringwhen used with an ordinary Otto-cycle internal combustion engine, and,even if it is ignited, the working efficiency (g/horse-power time) ofthe engine is greatly reduced because of its low combustion rate.

The stratified charge-type internal combustion engine having anauxiliary combustion chamber is an engine that overcomes the aforesaidshortcoming. In this engine, the combustion chamber comprises a maincombustion chamber and a smaller auxiliary combustion chamberintercommunicated therewith by a passage. While a lean air-fuel mixtureis introduced into the main combustion chamber, a rich air-fuel mixturethat is easy to ignite, is formed in the auxiliary combustion chamber bymeans of a fuel injection valve or an auxiliary intake valve. Thereadily ignitable rich mixture in the auxiliary combustion chamber isfirst ignited by a spark plug disposed therein. Then, the combustion gasproduced in the auxiliary combustion chamber is injected extensivelyinto the main combustion chamber through said passage whereby the leanair-fuel mixture in the main combustion chamber is ignited and combustedquickly. This increases the combustion rate in the main combustionchamber, and thus prevents the above-mentioned decreases in theignitability of the mixture and the engine efficiency. The developmentof this statified charge-type engine with such an auxiliary combustionchamber has advanced almost to the point that permits putting it intopractical use. However, the use of two types of air-fuel mixtures, thatis, lean and rich air-fuel mixtures, as described above, necessitates avery complex fuel supply system, which leads to such serious drawbacksas difficulties in manufacture, control and adjustment, increased costand troublesome maintenance and inspection work. Consequently, aninternal combustion engine that can use a lean air-fuel mixture, withoutrequiring a rich mixture, has been desired earnestly.

The inventor discovered that a lean air-fuel mixture can be igniteddirectly, that is, without employing a rich air-fuel mixture, if thespark plug or other igniting device is improved suitably. Also it wasdiscovered that, in an engine of the type that has no intake valve inits auxiliary combustion chamber, the waste gas that unavoidably remainsin the auxiliary combustion exerts a serious adverse influence on theigniting performance of the lean air-fuel mixture therein. Based onthese discoveries, the inventor previously proposed an engine that canbe operated with a lean air-fuel mixture, in which the formation of arich air-fuel mixture in the auxiliary combustion chamber is preventedby disposing the electrodes of the spark plug in the passage thatintercommunicates the main combustion chamber and the auxiliarycombustion chamber. In this engine, the waste gas, which remains in thepassage at the igniting time immediately before the completion of thecompression stroke, is moved into the auxiliary combustion chamber bythe flow of the mixture forced from the main chamber into the auxiliarychamber during the course of the compression stroke. As a result, thewaste gas is no longer present in the vicinity of the electrodes, thusimproving ignitability of the air-fuel mixture.

However, the engine of the above-described type, wherein the electrodesof the spark plug are located in the intercommunicating passage, also isnot free from problems. For example, the cross-section of theintercommunicating passage of this engine is made relatively small so asto provide the combustion gas with sufficient injection speed, in orderthat the effect of igniting and combusting the lean air-fuel mixture inthe main chamber quickly, known as the torch effect, is achieved to asufficient degree by causing the combustion gas in the auxiliary chamberto be injected powerfully into the extensive area of the main chamber.This reduction in the cross-sectional area of the passage, however,increases the speed of the mixture that flows through said passage, fromthe main chamber to the auxiliary chamber, at the time of ignition, andthis is liable to blow off the electric arc discharge between theelectrodes of the spark plug, resulting in misfiring. This problemapparently might be avoided by placing the spark plug electrodes in anarea where the mixture flows at a lower speed, that is, in theneighborhood of the wall of the passage. But it has experimentally beenproved that the placement of the electrodes near the passage wall lowersthe ignitability even more. The cause for this lowering of ignitabilityhas not been established clearly, but it seems to be possible that theflame core produced by the electric discharge is cooled down by thepassage wall. At any rate, the idea of disposing the electrodes in thevicinity of the passage wall is not satisfactory.

The object of this invention is to provide an internal combustion engineadapted for use with a lean air-fuel mixture, in which theabove-described problem experienced with the placement of the electrodesof the spark plug in the intercommunicating passage is solved.

This invention will be further described with reference to embodimentsthereof shown in the accompanying drawings.

Referring to the drawings:

FIG. 1 is a cross-sectional view of a fragment of a spark-ignition-typeinternal combustion engine having an auxiliary combustion chamber andthat embodies this invention.

FIG. 2 is a cross-sectional view of the spark plug portion of anotherembodiment of this invention.

FIG. 3 is a cross-sectional view showing another embodiment of thisinvention in which part of the embodiment of FIG. 2 is improved.

FIG. 4 is a cross-sectional view of a further embodiment of thisinvention.

FIG. 5 is a cross-sectional view showing a still further embodiment ofthis invention in which part of the embodiment of FIG. 4 is improved.

FIG. 6 is a cross-sectional view of yet another embodiment of thisinvention.

Referring to FIG. 1, there is shown a fragment of an internal combustionengine comprising a cylinder block 1, a cylinder 2 formed therein, areciprocatable piston 3 in the cylinder 2, and a cylinder head 4 mountedon the cylinder block 1. A main combustion chamber 6 is defined by thetop surface of the piston 3 at the top-dead-center position thereofduring the compression stroke and the internal surface of the cylinderhead 4. The main combustion chamber 6 is provided with an intake valve 5for introducing an air-fuel mixture and is also provided with an exhaustvalve (not shown) in a known fashion. Near the top of the maincombustion chamber 6 there is formed a recess 7 in the cylinder head 4,into which recess there is press-fitted an auxiliary combustion chambershell 8, defining an auxiliary combustion chamber 9 and a passage 10communicating said auxiliary combustion chamber 9 with the maincombustion chamber 6. The reason why the auxiliary combustion chamber 9is formed in the shell 8, which is a separate part from the cylinderhead 4, is that the auxiliary chamber 9 whose inside is exposed to avery high temperature can be made of a material that has a higher heatresistance than the cylinder head 4. Also, the press-fitting method isadopted for the purpose of preventing over-heating of the auxiliarychamber shell 8 by improving heat transfer to the cylinder head 4.Further, an opening 11, that communicates the inside of the passage 10with the outside of the cylinder head 4, is formed in both the auxiliarychamber shell 8 and the cylinder head 4, and a spark plug 12 is screwedtherein. The electrodes 12a of the spark plug 12 are disposed in thevicinity of the center of said passage 10. As illustrated, the insidediameter of the passage 10 is larger at a portion 10a where theelectrodes 12a are disposed than at a portion 10b that opens into themain combustion chamber 6. Briefly, the passage 10 has a largercross-sectional area at the portion 10a than at the portion 10b.

Because the portion 10a has a larger cross-sectional area than theportion 10b, the air-fuel mixture or the combustion gas always flows ata slower rate in the former portion than in the latter. The portion 10bdetermines the speed at which the combustion gas is injected into themain combustion chamber 6, and therefore the extent of the torch effect.Even if the cross-sectional area of this portion 10b is reduced toattain an increased torch effect, the increase in the flow speed of themixture, or the blow-off of the electric arc discharged between theelectrodes 12a, may be prevented if the cross-sectional area of theportion 10a is made larger.

FIG. 2 shows an embodiment in which the electrodes of the spark plug 12are disposed in an enlarged portion 10c of the passage 10, whichenlarged portion is located adjacent to and communicates with the maincombustion chamber 6. The passage 10 has a narrowed portion 10d whosecross-sectional area is smaller than said portion 10c and which islocated adjacent to and communicating with the auxiliary combustionchamber 9. In this embodiment, the cross section of the enlarged passageportion 10c is expanded on one side thereof by laterally offsetting wallportion 13A from wall portion 13B. The opposing wall 14 is straight.

In the embodiment of FIG. 2, the enlarged passage portion 10d has alength along the flow direction of passage 10 which is greater than theoutside diameter of the threaded portion of the spark plug. Thus, thegas velocity across the electrodes 12a is substantially reduced and theelectrodes 12a of the spark plug 12 are prevented from being exposed tothe swirl of air that occurs in the auxiliary combustion chamber 9.

FIG. 3 shows a modification of FIG. 2 in which the wall 14 has a taperedportion 14A adjacent to the main combustion chamber 6 and which tapersin a direction away therefrom toward the auxiliary combustion chamber 9.

In the embodiment of FIG. 3, the provision of a taper 14a permits asmooth supply of fuel-air mixture from the main combustion chamber 6 tothe auxiliary combustion chamber 9. As a consequence, a sufficientamount of air-fuel mixture can be fed into the auxiliary combustionchamber 9. Further, this taper 14 not only determines the direction inwhich the torch flame is projected, but also increases the freedom withwhich the auxiliary combustion chamber shell 8 (see FIG. 1) can befitted.

FIG. 4 shows an embodiment in which a recess 15 is provided in the wall13 of the passage 10, and the electrodes 12a are positioned in saidrecess 15. The recess 15 has a length along the flow direction ofpassage 10 which is greater than the outside diameter of the threadedportion of the spark plug. This results in the flow passage, in thevicinity of the recess 15, thus being of an enlarged crosssectionalarea, whereby the gasses which flow through this area and particularlythose gasses which flow in the vicinity of the spark plug electrodes arethus moving at substantially reduced velocity.

In this embodiment, the air-fuel mixture in the vicinity of theelectrodes 12a flows in a complex manner, which is considered to be dueto a decrease in the flow velocity caused by the combined effects of anincrease in the cross-sectional area of the passage 10 and thestagnation of flow in said recess. Despite such complicated flow,however, cooling of the electrodes 12a and blow-off of the spark areprevented, because the velocity of air-fuel mixture is reduced.

In the embodiment shown in FIG. 5, a taper 14a is provided on the end ofthat portion of the passage 10 shown in FIG. 4 which is adjacent to themain combustion chamber 6.

As described above with respect to the embodiment of FIG. 3, thisembodiment also permits supplying the air-fuel mixture into theauxiliary combustion chamber 9 smoothly.

In the embodiment shown in FIG. 6, the passage 10 is defined by twopassage portions which extend at an angle to each other. Moreparticularly, the passage 10 has a passage portion 10e having arelatively large cross-sectional area located adjacent the maincombustion chamber 6, and the passage 10 has a passage portion 10fhaving a relatively small cross-sectional area located adjacent theauxiliary combustion chamber 9. The angle α at which the center axes ofthe passage portions 10e and 10f intersect is selected to be as follows:

    - 90° < α < 90°

Here, the electrodes 12a are provided in that side of the wall of thepassage portion 10e that lies in the direction in which the center axisf--f of the passage 10f is turned from the center exis e--e of thepassage portion 10e at said angle α, that is, on the outside of thecurve or bend.

In this embodiment, the air-fuel mixture urged into the auxiliarycombustion chamber 9 changes flow direction or is deflected because thepassage 10 is bent. Therefore, centrifugal force acts to bring a richer(heavier) portion of the air-fuel mixture close to the electrodes 12a,thereby facilitating ignition.

To reduce the noxious components of the exhaust gas from an engine ofthe above-described type, it is desirable to make the ratio of air tofuel high throughout all engine load conditions. But combustion of alean air-fuel mixture is unavoidably accompanied by a drop in engineoutput or torque so it is desirable to lower the air-fuel ratio in thehigh-load region in which a high output is demanded, for example, duringengine acceleration.

It is accordingly desirable to improve the responsiveness of the enginein the high-load region by controlling the air-fuel ratio to be at ahigh level in the medium (steady speed) and low load (idling) regions,and at a low level in the high load (acceleration) region.

The period of time during which the engine is operated under high loadis small compared with the total operation time, and therefore anincrease in exhaust emissions during such time is very small, too. So,the advantage offered by the improved responsiveness of the engine isgreater than the disadvantage resulting from an increase in exhaustemissions.

Thus, this invention provides an internal combustion engine suited foruse with a lean air-fuel mixture, which is simple in construction andcapable of eliminating the previously described problem that hasheretofore been unavoidable with the spark plug electrodes disposed inthe intercommunicating passage.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. In a spark-ignition-type internal combustion engine having a main combustion chamber equipped with an intake valve for an air-fuel mixture, an auxiliary combustion chamber, a transfer passage interconnecting said main combustion chamber with said auxiliary combustion chamber, and a spark plug associated with the transfer passage, the spark plug having an annular threaded portion mounted in a wall defining the transfer passage so that the spark plug electrodes project into said passage, whereby the air-fuel mixture is forced during the compression stroke from the main combustion chamber into the auxiliary combustion chamber and is ignited by the spark plug electrodes, the combustion gas in the auxiliary combustion chamber is injected through said passage into the main combustion chamber, and thus the air-fuel mixture in the main combustion chamber is quickly ignited and combusted, comprising the improvement wherein said transfer passage has a first passage portion of enlarged cross-sectional area and at least one additional passage portion of a smaller cross-sectional area, said electrodes being positioned in said first portion of said transfer passage, and said first portion having a length as measured in the flow direction of the passage which is greater than the outside diameter of the threaded portion of the spark plug.
 2. A spark-ignition-type internal combustion engine as set forth in claim 1, wherein the air-fuel mixture supplied to the main combustion chamber is lean when the engine load is medium and low, and is rich when the engine load is high.
 3. A spark-ignition-type internal combustion engine as set forth in Claim 1, wherein the auxiliary combustion chamber is defined by an auxiliary combustion chamber shell which is press-fitted into a recess in the cylinder head from the main combustion chamber side.
 4. A spark-ignition-type internal combustion engine as claimed in claim 1, in which said first portion of enlarged cross-sectional area is an intermediate portion of said passage, and said passage has end portions on opposite sides of said first portion, at least one of said end portions being of smaller cross-sectional area than said intermediate portion, the velocity of flow of the gas being reduced in said first portion of said passage adjacent said electrodes in an amount effective to avoid cooling of the electrodes and quenching of the spark.
 5. A spark-ignition-type internal combustion engine as claimed in claim 4, in which said one end portion of said passage is the portion between said main combustion chamber and said intermediate portion.
 6. A spark-ignition-type internal combustion engine as claimed in claim 4, in which said one end portion of said passage is the portion between said auxiliary combustion chamber and said intermediate portion.
 7. A spark-ignition-type internal combustion engine as claimed in claim 6 wherein the other end portion of said passage tapers in a direction toward said one end portion.
 8. A spark-ignition-type internal combustion engine as claimed in claim 1 in which said passage is defined by a peripheral wall having an outwardly offset wall portion, or a recess, defining in conjunction with the remainder of the passage said first portion of enlarged cross-sectional area, the electrodes of said spark plug being entirely disposed within said offset portion or said recess, outside of the remainder of the passage and spaced inwardly from the passage wall.
 9. A spark-ignition-type internal combustion engine as claimed in claim 4, in which both end portions of said passage are of smaller cross-sectional area than said intermediate portion and wherein said electrodes are positioned entirely within said first portion of enlarged cross-sectional area of said passage.
 10. A spark-ignition-type internal combustion engine as claimed in claim 6, in which said intermediate portion and said one end portion extend at an angle to each other and the spark plug electrodes are located on the outside of the included angle defined by the axes of said portions.
 11. A spark-ignition-type internal combustion engine according to claim 1, wherein the wall of said transfer passage includes a first wall portion which comprises one of the boundaries of said first passage portion, the wall of said passage including a second wall portion which defines one of the boundaries of said additional passage portion, said additional passage portion being located between said first passage portion and said auxiliary combustion chamber, said second wall portion being offset inwardly from the first wall portion so as to be in closer positional relationship with respect to the longitudinally extending centerline of the transfer passage, and the wall of said transfer passage including an intermediate wall portion which extends transversely between and joins said first and second wall portions, said intermediate wall portion defining a shoulder which is disposed directly downstream of said electrodes when the flow occurs through the passage in a direction from the main combustion chamber into the auxiliary combustion chamber. 